The present invention relates to the detection of lens aberrations associated with the projection lens utilized in a lithography system and more particularly to the design, layout and application of lens-aberration monitoring structures that can be used to monitor the projection lens performance during the manufacture of semiconductor devices.
Lithographic apparatus may employ various types of projection radiation, non-limiting examples of which include ultra-violet light (xe2x80x9cUVxe2x80x9d) radiation (including extreme UV (xe2x80x9cEUVxe2x80x9d), deep UV (xe2x80x9cDUVxe2x80x9d), and vacuum UV (xe2x80x9cVUVxe2x80x9d)), X-rays, ion beams or electron beams. Depending on the type of radiation used and the particular design requirements of the apparatus, the projection system may be for example, refractive, reflective or catadioptric, and may comprise vitreous components, grazing-incidence mirrors, selective multi-layer coatings, magnetic and/or electrostatic field lenses, etc; for simplicity, such components may be loosely referred to in this text, either singly or collectively, as a xe2x80x9clensxe2x80x9d.
In a manufacturing process using such a lithographic projection apparatus, a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the images features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g., an integrated circuit (IC). Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes may be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997 ISBN 0-07-067250-4.
The current state of integrated circuit (IC) fabrication requires lithography processes to provide for patterning feature line widths to near one-half of the exposure wavelength. For the 150 nm device generation, the krypton fluoride (KrF) excimer laser (248 nm) is typically selected as the exposure source of choice. Recent research and development efforts have further demonstrated the possibility of utilizing the KrF excimer laser for the 130 nm device generation. This is achieved by combining the use of multiple resolution enhancement techniques (RET), such as, attenuated phase-shifting masks (attPSM) and off-axis illumination (OAI), in combination with optical proximity correction (OPC) techniques. One possible alternative to the foregoing techniques is to use a shorter exposure wavelength, for example, an argon fluoride (ArF) excimer laser having a wavelength of 193 nm. However, due to various complications associated with the use of the ArF excimer laser, it is likely that the KrF excimer laser will be the dominant laser of choice for fabricating the 130 nm device generation.
Regardless of the excimer laser utilized in the fabrication process, the fabrication of devices having critical dimensions of 150 nm or less requires that the near-diffraction-limited lens utilized in the fabrication process be substantially aberration free. As is known, aberrations can be caused by various sources, such as a defective lens or an aging laser which emits a beam having a frequency shifted from the desired value. Accordingly, it is desirable to verify lens performance (i.e., qualify the lens) prior to installation, and then to subsequently monitor the lens performance during use (e.g., in an IC fabrication process).
During the lens manufacturing process, the lens performance can be fully tested interferometrically. Typically, the lens is first qualified at the factory and then again during the initial installation in the field. One common practice utilized for lens qualification is to print wafers and then measure the dimensions of the minimum feature width, or the critical dimension (CD). During this qualification process, both xe2x80x9cverticalxe2x80x9d and xe2x80x9chorizontalxe2x80x9d features are measured (i.e., features extending in two orthogonal directions on the substrate plane). In some instances, the CD for 45-degree features is also measured. In order to verify lens performance, a sufficient number of CD measurements are required across the entire exposure field. The results of the CD measurements are then analyzed to determine whether or not the lens performance is acceptable.
Although the CD measurement method provides a method of evaluating the performance of the lens, it is not a simple task to correlate the CD data to the xe2x80x9csignaturexe2x80x9d of the lens aberration. Accordingly, there have been efforts to perform a direct observation of lens aberrations. For example, an article by Toh et al. entitled xe2x80x9cIdentifying and Monitoring of Lens Aberrations in Projection Printing,xe2x80x9d SPIE Vol. 772, pp. 202-209 (1987) described methods for measuring the effects of relatively large lens aberrations of about 0.2xcex, where xcex is the exposure wavelength. However, for today""s near-diffraction-limited optics, any lens aberration is likely to be in the neighborhood of 0.05xcex, or smaller. For 130 nm features, a 0.05xcex lens aberration translates to a 12.4 nm dimensional error when utilizing the KrF exposure source. Accordingly, if the feature CD budget (i.e., error tolerance) is assumed to be xc2x110% of the target feature width, a 12.4 nm error consumes almost the entire CD budget.
In an article by Gortych et al. entitled xe2x80x9cEffects of Higher-Order Aberrations on the Process Window,xe2x80x9d SPIE Vol. 1463, pp. 368-381 (1991) it was demonstrated that higher-order lens aberrations could deteriorate lithographic process windows. Unfortunately, the higher-order lens aberrations are difficult to eliminate after the photolithography system is assembled. In an article by Brunner entitled xe2x80x9cImpact of Lens Aberration on Optical Lithography,xe2x80x9d INTERFACE 1996 Proceedings, pp. 1-27 (1996) simulation was utilized to demonstrate the negative impact of near-wavelength features due to several first-order lens aberrations. Specifically, it was possible to observe coma aberrations by examining how the contact features were printed when utilizing attenuated PSM. It is also known that that lens aberrations can be balanced with custom off-axis illumination. Others have attempted to make direct measurements of various kinds of lens aberrations in an effort to achieve better CD control.
An article by Farrar et al. entitled xe2x80x9cMeasurement of Lens Aberrations Using an In-Situ Interferometer Reticle,xe2x80x9d Advanced Reticle Symposium, San Jose, Calif. (June 1999) reported the use of an in-situ interferometer reticle to directly measure lens aberration. According to Farrar, it was possible to derive lens aberrations up to 37 Zernike terms. Although Farrar claims that the method is accurate and repeatable, it involves hundreds or thousands of registration type measurements (i.e., the measuring of the shift in relation to the intended feature position). As such, while Farrar""s method may be accurate and repeatable, with the need for such exhaustive measurements, the method is clearly very time consuming, and therefore likely unusable in a manufacturing-driven environment. Furthermore, it is conceivable that minute lens aberrations can drift over time due to various reasons (e.g., as a result of the periodic preventive maintenance performed on a system). Thus, as it is critical to monitor lens performance on a periodic basis, the use of Farrar""s method, which requires substantial measurements and calculations, is impractical. Accordingly, there is a need to be able to monitor the lens aberration directly from the printed product wafers.
In an effort to accomplish this objective, in 1999 Dirksen et al. (see, U.S. Pat. No. 6,248,486, filed Sep. 29, 1999, incorporated herein by reference) proposed a method for directly monitoring lens aberration from the printed wafers. According to Dirksen""s method, the lens monitor comprises simple circular features on the reticle. More specifically, the circular feature is a chromeless feature that has been etched into the glass substrate of the reticle. The etched depth is typically xcex/2 and the diameter is about (xcex/NA), where NA is the numerical aperture of the projection lens. According to Dirksen, the method has proven to be effective. Further, the structure is simple and small enough to be readily placed throughout the entire exposure field.
Still, there are a number of issues concerning the use of Dirksen""s lens aberration monitor. First, the depth of the lens monitor feature on the mask needs to be etched to approximately half of the wavelength. For a special-purpose mask, there is no problem dedicating an extra (or special) mask making process step to fabricate such a feature. However, for production reticle types, such as a binary chrome reticle or attPSM, an extra mask making process step necessary to create such a monitor is costly and time-consuming process. Alternating PSM (altPSM) or chromeless PSM (CLM) would also require the extra mask making process step. Further, since the Dirksen monitor calls for a different etch depth in the quartz substrate as opposed to the xcfx80-phase, it requires a special etch time and must be done separately.
A second issue with Dirksen""s lens monitor is that it is vulnerable to phase error that may result from the quartz etch process during mask formation. More specifically, referring to FIGS. 1(a)-1(f), for an exacerbated phase error, the quartz etch process causes a sloped edge profile on the mask as shown in FIG. 1(a). In such a case, the Dirksen monitor loses all of the sensitivity to indicate any possible lens aberration. However, when there is no phase-error on the mask, as shown in FIG. 1(d), the Dirksen monitor is effective for detecting lens aberrations. FIGS. 1(b) and 1(e) illustrate a cross-sectional view of the printed resist pattern resulting from the xe2x80x9cslopedxe2x80x9d Dirksen monitor structure of FIG. 1(a) and the xe2x80x9cidealxe2x80x9d Dirksen monitor structure of FIG. 1(d), respectively.
It is noted that the printing conditions utilized to produce the resist profiles illustrated in FIGS. 1(b) and 1(e) were as follows: a 0.68 NA with 0.8 partial coherence at +0.1 xcexcm de-focus, utilizing a Shipley UV6 resist with a thickness of 0.4 xcexcm on an organic BARC (AR2) on top of a polysilicon wafer. The simulation introduced a +0.025xcex coma for both X and Y (Z7 and Z8 of Zernike terms).
Upon a closer examination of the ring-shaped resist patterns formed by the Dirksen monitor structures, as shown for example in FIGS. 1(c) and 1(f), it is clear that the inner ring of the printed resist pattern has a relatively sloppy resist profile in contrast to the steep profile formed by the outer ring structure. The reason for this variation is that the outer-ring resist pattern is formed by the phase change in the mask, while the inner ring resist pattern is formed without any such phase change. Specifically, the inner ring resist pattern is formed via the attenuation of the exposure wavelength that is passed through the center of the Dirksen monitor pattern. In other words, the two resist profiles (i.e., the inner ring and the outer ring) are formed by two inherently different log-slopes of the respective aerial images. The difference in resist profiles can lead to erroneous registration measurements, which can cause a misinterpretation of the lens aberration in question.
It is noted that it is possible to observe a slight coma with the Dirksen lens aberration monitor, as shown in FIGS. 1(e) and (f). Specifically, the width of the ring is different on the left side as compared to the right side. It is further noted that it is difficult to observe this coma in the xe2x80x9cslopedxe2x80x9d Dirksen monitor, as shown in FIGS. 1(b) and 1(c).
Accordingly, in view of the foregoing problems, there remains a need for a lens monitor that allows for the detection of lens aberrations, but which is not easily impaired by slight imperfections in the mask making process. It is also desirable that the lens monitor structures be small enough such that they can be positioned in numerous places between or beside production die for in-situ lens monitoring purposes. It is also desirable that the lens monitor can be fabricated without requiring extra mask making process steps.
In an effort to solve the aforementioned needs, it is an object of the present invention to provide a lens monitor capable of detecting lens aberrations, where the lens monitor structures are sufficiently small in size so as to allow the monitor to be utilized for in-situ production monitoring, and which monitor does not require extra processing steps during mask formation. In addition, the functionality of the lens monitor should not be significantly impaired by minor imperfections in the mask formation process.
More specifically, the present invention relates to a lens aberration monitor for detecting lens aberrations. The lens aberration monitor comprises a mask for transferring a lithographic pattern onto a substrate, and a plurality of non-resolvable features disposed on the mask. The plurality of non-resolvable features are arranged so as to form a predetermined pattern on the substrate. The predetermined pattern is then utilized to detect lens aberrations. The size of the monitor is such that the mask can also contain a lithographic pattern corresponding to a device (e.g., an integrated circuit) to be formed on the substrate.
The present invention also relates to a method of detecting aberrations associated with a projection lens utilized in an optical lithography system. The method comprises the steps of forming a mask for transferring a lithographic pattern onto a substrate, forming a plurality of non-resolvable features disposed on the mask, where the plurality of non-resolvable features are arranged so as to form a predetermined pattern on the substrate, imaging the mask using the optical lithography system so as to print the mask on the substrate, and analyzing the position of the predetermined pattern formed on the substrate and the position of the plurality of non-resolvable features disposed on the mask so as to determine if there is an aberration. As explained below, if the position of the predetermined pattern differs from an expected position, which is determined from the position of the plurality of non-resolvable features, this shift from the expected position indicates the presence of an aberration.
As described in further detail below, the present invention provides significant advantages over the prior art. Most importantly, the present invention provides a lens monitor capable of detecting very subtle lens aberrations, and is substantially immune to deficiencies in the masking formation process utilized to form the monitor.
In addition, the lens monitor of the present invention is suitable for in-situ monitoring, as the lens monitor can be formed utilizing the same mask formation process required to form the production mask, and therefore does not require any additional mask formation processing steps. Furthermore, as the overall size of the lens monitor structures are sufficiently small, the monitor structures can be positioned in a sufficient number of positions in so as to allow for monitoring of the entire exposure field.
Yet another advantage is that the effectiveness of the lens monitor is relatively insensitive to both of the xe2x80x9cslopedxe2x80x9d phase edges and the xe2x80x9ccorner roundingxe2x80x9d effects that are inherent to mask making process.
Additional advantages of the present invention will become apparent to those skilled in the art from the following detailed description of exemplary embodiments of the present invention.